Following birth, dogs show a rapid decline in blood hemoglobin concentration which is defined as postnatal anemia. We have shown that the total hemoglobin does not change during this period; however the rapid growth of the animal, including its vascular system, causes a drop in hemoglobin concentration. There is a change in hemoglobin function during this period effectively allowing a given quantity of hemoglobin to transport more oxygen. This change in function is mediated by an intracellular increase in 2,3-diphosphoglycerate concentrations. The mechanism of the 2,3-diphosphoglycerate changes is the current focus of our work. We are measuring alterations in canine erythrocyte glycolysis including levels of glycolytic intermediates and enzymes at 10 day intervals from birth to 120 days of life. These alterations will be analyzed to determine the in vivo mechanism of 2,3-diphosphoglycerate regulation in the postnatal dog. We are also examining the capacity of the neonatal dog to produce hemoglobin and red cells during the period where there is no change in total hemoglobin in spite of the rapid growth of the animal.